Specific examples of a conventional high-pressure discharge lamp include the high-pressure mercury vapor discharge lamps disclosed in Japanese Laid-Open Patent Publication No. 2-18561 and Japanese Patent Publication No. 2980882. FIG. 13 schematically shows a structure of the high-pressure discharge lamps disclosed in these publications. Hereinafter, the conventional high-pressure discharge lamp 1000 will be described with reference to FIG. 13.
The lamp 1000 includes a luminous bulb (bulb) 110 made of quartz glass and a pair of sealing portions (seal portions) 120 coupled to both ends of the luminous bulb 110. Inside the luminous bulb 110 (discharge space), a pair of tungsten electrodes (W electrodes) made of tungsten are opposed to each other with a predetermined distance. One end of the W electrode 112 is welded to a molybdenum foil (Mo foil) 124 in the sealing portion 120, so that the W electrodes 112 and the foils 124 are electrically connected. An external lead (Mo rod) 130 made of molybdenum is electrically connected to one end of the Mo foil 124.
Japanese Laid-Open Patent Publication No. 2-18561 discloses a high-pressure discharge lamp having the same structure as shown in FIG. 13 in which a rare gas, 200 mg/cc or more of mercury 118 and halogen in the range of 10−3 to 10−1 μmol/cc are enclosed in the luminous bulb 110 and which is operated with a bulb wall load of 100 W/cm2 or more.
Japanese Laid-Open Patent Publication No. 2-18561 describes the following. When the bulb wall load is increased to 100 W/cm2 or more, the temperature at the coldest point can be increased. Therefore, when mercury is enclosed in an amount 200 mg/cc or more, the pressure in the luminous bulb 110 is increased, and continuous spectrum, especially in a red region, is increased so that the color rendering property can be improved. In addition, it is described that the reason why halogen is enclosed is to prevent the blackening of the luminous bulb 110, but there is no specific description of a clear reason why the amount of halogen enclosed is defined to be in the range of 10−3 to 10−1 μmol/cc.
On the other hand, Japanese Patent Publication No. 2980882 discloses a high-pressure discharge lamp having the same structure as shown in FIG. 13 in which a rare gas, 160 mg/cc or more of mercury 118 and halogen in the range of 2×10−1 to 7 μmol/cc are enclosed in the luminous bulb 110 and which is operated with a bulb wall load of 80 W/cm2 or more.
Japanese Patent Publication No. 2980882 describes the following speculation. When the discharge lamp is operated at a bulb wall load of 80 W/cm2 or more, the temperature of the luminous bulb 110 is increased so that the absorption wavelength band of quartz glass is shifted to the long wavelength side. Therefore, mercury-rare gas excimer light with a wavelength of 185 nm (which is light radiated by discharge in a mixed gas of a high-pressure mercury vapor and a rare gas when mercury 118 is enclosed in an amount of 160 mg/cc or more) easily can be absorbed. As a result, opaqueness and devitrification of the quartz glass are grown rapidly. Furthermore, it is described that Si or SiO is released from the quartz glass that has absorbed ultraviolet rays with a short wavelength, and they are attached to the heads of the electrodes 112 so that the melting point of tungsten is reduced, so that deformation of the electrodes 112 or blackening of the bulb wall of tungsten occur.
Regarding this problem, Japanese Patent Publication No. 2980882 describes that the opaqueness and the devitrification of the quartz glass are prevented by enclosing halogen in the range of 2×10−1 μmol/cc or more to let the halogen absorb the excimer light. It is also described that the halogen is enclosed in an amount of 7 μmol/cc or less in order to prevent the deformation of the electrodes 112. Furthermore, it is disclosed that in order to suppress the growth of the opaqueness and the devitrification, the average concentration of OH groups on the inner surface (a depth of within 0.2 mm) of the quartz glass is defined to 20 ppm or less.
Moreover, this publication describes the following. Carbon atoms are attached on the inner surface of the luminous bulb 110 during operation of the lamp and absorb ultraviolet rays, which consequently promotes the opaqueness and the devitrification. Therefore, halogen is enclosed in the luminous bulb 110 in the form of not a compound containing carbon (CH2Br2 etc), but, for example, hydrogen bromide (HBr). Furthermore, it is described that the higher the ratio of aluminum and alkali metals (sodium, potassium, and lithium) in the quartz glass is, the higher the viscosity of the glass is, so that the speed of the opaqueness and the devitrification (crystal growth of SiO2) is reduced. Then, it is described that such a specification can provide a high-pressure discharge lamp having a lifetime of about 2000 hours, which is required as a liquid crystal projector.
In the conventional high-pressure discharge lamp 1000, as shown in the above publications, the blackening or the opaqueness and the devitrification of the luminous bulb 110 are prevented by halogen.
The inventors of the present invention produce high-pressure discharge lamps containing various kinds of halogen for testing based on the techniques and the specifications disclosed in the above publications, and a lifetime test was conducted by incorporating the produced lamps in reflecting mirrors, and changes in the illuminance of a screen such as a liquid crystal projector were examined. Then, it was found that the illuminance of the screen was significantly reduced in a lamp operation time of about several tens hours to several hundreds hours. In particular, the obtained results showed that for high output lamps with 50 W or more, which are not disclosed in Japanese Laid-Open Patent Publication No. 2-18561, the illuminance reduction was significant. Furthermore, it was found that in these conventional lamps, as the operation time becomes longer, the voltage at the start of discharge becomes higher, so that the lamps become difficult to operate.
As a result of observing these lamps in detail, the inventors of the present application learned the following. First, in an early time immediately after the operation, the tungsten of the electrode 112 is attached to the inner surface of the luminous bulb 110 so that the luminous bulb 110 is blackened (blackening). Then, the blackening proceeds rapidly, and the blackening causes the transmittance of the luminous bulb 110 to be reduced significantly. Consequently, the qunatity of light emitted from the luminous bulb 110 is reduced. These results of observation indicate that the blackening or the opaqueness and the devitrification of the luminous bulb 110 cannot completely be prevented even if halogen is enclosed in the luminous bulb 110 only by using the techniques and the specifications disclosed in the above publications. Therefore, In the conventional lamp 1000 disclosed in the above publications, the optical output is reduced in an early time after the lamp is operated.
In a liquid crystal projector that projects a screen of a personal computer in a larger scale (so-called data projector), it seems that when the conventional lamp 1000 of this kind was used, the short lifetime of the lamp was not very large problem. This is because in such applications, in most cases, the projector was used for general public for a relatively short time in such occasions as conferences in companies or schools, so that there was substantially no need of exchanging lamps frequently, and therefore there was no burden on the user.
However, when the conventional lamp 1000 is used for television (projection television) for ordinary homes, the short lifetime of the conventional lamp 1000 is a large problem. That is to say, at the present when TV programs of several tens of channels are broadcast in ordinary homes, as typified by digital broadcasting, there is an increasing demand for enjoying these broadcast programs with a multiscreen or having a large amount of character information displayed together with images in a large screen television at home. To meet this demand, the development to apply the data projector that can provide large screen images to television for ordinary homes (projection television) has been started full-fledgedly. In this application, television is used for many hours a day, so that the operation time of the lamp is naturally longer than that when used in the data projector. Furthermore, it is in demand that exchange of lamps is unnecessary or is limited to a number of times as small as possible. In order to meet this demand, the lifetime of the lamp is required to be at least 5000 hours to 100000 hours or more. However, the conventional lamp 1000 cannot provide the lamp lifetime of 5000 hours to 100000 hours or more, which is required by projection type liquid crystal televisions.
The present invention is carried out in view of these points, and it is a main object of the present invention to provide a high-pressure discharge lamp having a long life.